Add argument types tait tests

src/test/ui/type-alias-impl-trait/argument-types.rs

@@ -0,0 +1,28 @@
+#![feature(type_alias_impl_trait)]
+#![allow(dead_code)]
+
+use std::fmt::Debug;
+
+type Foo = impl Debug;
+
+// FIXME: This should compile, but it currently doesn't
+fn foo1(mut x: Foo) {
+    x = 22_u32;
+    //~^ ERROR: mismatched types [E0308]
+}
+
+fn foo2(mut x: Foo) {
+    // no constraint on x
+}
+
+fn foo3(x: Foo) {
+    println!("{:?}", x);
+}
+
+fn foo_value() -> Foo {
+    11_u32
+}
+
+fn main() {
+    foo3(foo_value());
+}

src/test/ui/type-alias-impl-trait/argument-types.stderr

@@ -0,0 +1,15 @@
+error[E0308]: mismatched types
+  --> $DIR/argument-types.rs:10:9
+   |
+LL | type Foo = impl Debug;
+   |            ---------- the expected opaque type
+...
+LL |     x = 22_u32;
+   |         ^^^^^^ expected opaque type, found `u32`
+   |
+   = note: expected opaque type `impl Debug`
+                     found type `u32`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.